osTicket email templates can reference ticket/message data by %{some_property} variables.

There is no official template variables list available in osTicket wiki. One of the best approximations I have found is

osTicky supports most relevant template variables listed in this document. By 'relevant' I mean that osTicky component functionality is limited (by the project natue) to frontend ticket/thread actions, i.e. create tickets and post replies in ticket thread. That is why there is no need to support variables related with other actions, i.e. internal notes, ticket assignment, etc. osTicky will only use the following templates:

osTicky works OK with predefined osTicket templates. If you modify a template and find a template variable not supported, please open a ticket.

Custom fields have a good template variables support in osTicky. You have to reference custom fields including the field "variable name" into %{ticket.variable_name} pattern. Please note, that if you add more than one custom form to your tickets you have to use unique variable names for all fields, even if they reside on different forms, otherwise their behavior in email templates will be undefined. Custom lists properties have slightly different template variables pattern.